PROJECT SUMMARY While metastasis is the cause of >90% of cancer patient deaths, the majority of targeted therapies are developed based on the biology of the primary tumor. This discrepancy highlights the pressing clinical need to better understand what gives cancers the ability to colonize and thrive within a new microenvironment. For osteosarcoma (OS), there are no targeted anti-metastatic therapies, despite nearly all patients presenting with some degree of metastatic burden. Recent studies from our laboratory have demonstrated the process of OS lung metastasis is driven by alterations in the enhancer landscape. Metastasis-specific variant enhancer loci (met-VELs) are ubiquitous, and regulate genes critical for growth of this primary bone cancer within the lung. However, the heterogeneity of OS has limited the clinical translation of these findings. A unifying molecular mechanism upstream of the enhancer remodeling associated with metastasis will therefore provide novel nodes for therapeutic targeting. Combining comprehensive enhancer analysis with in vivo functional genomic screening, I have identified the pluripotency factor KLF4 as enriched at met-VELs and critical for lung metastasis. The goals of this proposal are to determine if KLF4 is necessary and sufficient for OS metastasis through its role in maintaining and generating stem cell-like enhancer chromatin. Successful completion of this study will not only reveal new therapeutic targets and prognostic biomarkers, but may also inform the treatment of patients suffering from other metastatic cancers.